The electron density of the ionosphere varies as a function of geographic location and time. The International Civil Aviation Organization (ICAO) is developing standards to address the threat to navigation and guidance systems in aircraft stemming from an ionosphere anomaly, i.e., extreme changes in electron density of the ionosphere. The standard development has shown it is critical that ground monitoring for delay gradients in signals due to an ionosphere anomaly is absolute and not relative to previous satellite measurements. It is important to detect an extreme gradient from the very first set of measurements taken from a satellite as it moves into view of a ground-based GPS augmentation system.
In a ground station, carrier phase measurements from reference receiver pairs that are inconsistent with one another due to carrier wavelength ambiguity, can lead to missed detections or incorrect re-admittance of satellite data. For example, if the carrier phase measurement for one reference receiver pair is near ±λ/2, then noise on this reference receiver pair may result in a carrier phase measurement of −λ/2 while a combination of other reference receiver pairs projected along the baseline of the first reference receiver pair may result in a carrier phase measurement of +λ/2. In this case, the carrier phase measurements, cancel each other in a currently available gradient monitor.